(i) Field of the Invention
This invention relates generally to methods for backfilling undergrounds void in mining or similar applications, and, more particularly, to a method for filling stopes in underground ore mining operations.
(ii) Related Art
In many underground ore mining operations it is common to mine underground by forming a vertical lift shaft or helical tunnel and then to mine horizontally extending tunnels from the central axis at different levels. Large volumes of ore may then be removed via the horizontal tunnels by blasting a succession of stopes or underground voids upwardly from the far end of each tunnel progressively back toward the center axis. In order for safe mining to proceed it is necessary to backfill each underground void or stope formed as part of the blasting and ore evacuation procedure. This is necessary to support the "roof" above the stope and to allow an immediately adjoining volume of ore to be blasted in safety.
The mine backfilling is typically carried out by mixing a suitable particulate solid material, typically the mine tailings, with cement and water and conveying, trucking or pumping the backfill mixture to the location of the underground stope. Excess water draining from the backfill mixture must be pumped from the mine and the backfill mixture allowed to set to for a solid fill in the stope.
The cost of backfilling is significant and can be as much as 20% of the total cost of the mining operation. The cost of backfilling is directly related to the cost of the cement content in the filling mixture but a significant cost is also involved in transport of the material to the void. The most convenient way of transporting the material to the void is by pumping through pipes but this requires a significant water content in the backfill mixture. A conflicting requirement is that, in order to avoid mud slides underground (i.e., the fluidic collapse of part of the backfilled material) the recommended percentage of solids in the fill is above 74%. It is often difficult to pump such a mixture (at this ratio of solids) significant distances but any increase in water content to improve pumpability increases the risk of mud slides and increases the volume of cement required in the mix to reach specified strengths which are typically in the order of 1 MPa.
The amount of cement varies according to the backfill material and the water content but is usually around 6% in order for the fill to reach the required strength.
It is desirable to be able to provide a backfill material which is easy to pump and therefore economic to place without requiring high water content which increases the risk of mud slides or high cement content which increases the cost of the operation. Such filling material is also desirable for use in other mining situations such as the filling-in of disused mines to remove hazardous threats from cave-ins in subsequent open cast mining operations and other similar situations.